Although solar ultraviolet radiation is a proven mutagen, toxicant, and carcinogen, little is known about the underlying cellular and molecular processes causing these effects in mammalian cells, especially by the longer ultraviolet wavelengths. Using human cells (P3 teratocarcinoma) in culture and an isolated fragment of human DNA, we propose (1) to study the spectral efficiencies for specific DNA alterations (breakage, protein cross-links, pyrimidine dimers, and (6-4) pyrimidine adducts) caused by wavelengths longer than 290 nm, and (2) to attempt to correlate the specific DNA lesions observed with mutagenesis and cell killing caused by solar uv. Isolated wavelength bands and lines within the 290 to 435 nm range as well as the complete spectrum of terrestrial solar radiation itself (290 through visible light) will be used. The effects of dose modifying reagents, especially oxygen, upon the biological and biophysical end points will be studied. These experiments will provide insight into basic molecular mechanisms of the biological action of solar ultraviolet light, and provide information relevant to our understanding of hazards resulting from possible attenuation of our stratospheric ozone shield.